The present invention relates to torque transmitting devices, such as torque limiting clutches and torque sensors in which the torque load is transmitted through rolling elements, i.e. balls or rollers, arranged in a manner that is sensitive to unduly large torque loads.
The use of rolling elements in the form of balls or rollers in torque transmitting devices is well known, and, more particularly, such elements are known in overload release clutches. Examples are to be found in UK Pat. Nos. 846,450 and 881,560, both of which shown mechanisms in which two series of rollers mounted in radially inner and outer members respectively transmit the drive between said members. These clutches are released by allowing the rollers of the respective series to move past each other when a maximum loading is exceeded, but although such displacements are accompanied by some rolling movements, it is inherent in the arrangement of the rollers that they skid with respect to surfaces carrying some of the main reaction forces from the rollers and, accordingly, a significant amount of friction is experienced.
Another example of a clutch using balls as torque transmission elements is disclosed in UK Pat. No. 591,429. In this the mechanism comprises one series of balls transmitting the torque between driving and driven members, and a further inner series of balls bearing on the first series to hold them in their driving positions. The two series of balls are axially offset from each other so that the load on the torque-transmitting balls also produces a force component urging the balls of the second series axially away. Such axial movement is normally restrained but with an excessive rise in the torque load being transmitted, they are allowed to move away and the first series of balls are released from engagement between the driving and driven members. Here also there are major forces at points of sliding contact of the balls during the release action, as the second series of balls are held in a cage and are squeezed between the first series of balls and the cage to move, with their cage, axially of the coupling.
Such frictional effects are also experienced in the construction disclosed in UK Pat. No. 1,048,125 where a number of torque-transmitting rollers held axially against oblique faces of a driven member are required to slide on all their points of bearing contact to release the coupling. The release mechanism in this instance employs a thrust-applying device comprising a series of balls that are themselves constrained to slide on bearing points through which the main forces on them act, so that the frictional effects come from two different sources in the release of this form of coupling.
Another overload release coupling with single series of balls transmitting torque is to be found in UK Pat. No. 643,528. One of the balls of the series is used with an additional ball as a supplementary thrust-applying device having the purpose of operating a drive rotor cut-out mechanism when the coupling is overloaded and here also frictional effects are relatively high in relation to the force applied.
The frictional effects referred to above are of considerable significance in the operation of an overload release clutch. It is normally important that the release load shall be reliably and accurately preset in such a clutch, since failure to release when required might lead to damage through overloading the drive, while release at too low a load may cause unnecessary and wasteful stoppages. If frictional effects predominate, the presetting of the release load value becomes more uncertain and this is particularly so in the common circumstance where the release mechanism is required to operate only very occasionally. In that case, it becomes even more difficult to ensure the state of lubrication at points of sliding contact where the contacting surfaces do not move relative to each other while the mechanism is operating normally. The coefficient of friction may vary with time as any lubricant originally at the points of contact is gradually forced out due to the maintenance of high bearing pressures and the lack of relative movement at these points hinders the admission of further lubricant.
Although they are not concerned with torque transmission systems, reference may also be made to UK Pat. Nos. 483,145 and 488,598 which show drum brake mechanisms in which an operating thrust to expand the brake is applied by the displacement of a plunger which carries a pair of balls or rollers side-by-side between a pair of obliquely convergent abutment members attached to the brake shoes, so that the plunger acts in the manner of a wedge to force the members apart.
To explain the action of these devices, reference may be made firstly to FIG. 1 of the accompanying drawings which show a sliding wedge-type thrust applying device in which a wedge 1 applies a thrust F to two inclined planar surfaces 2 and 2A forcing them apart in a direction transverse to the force F. The resulting side thrust Q is accompanied by a frictional force .mu.Q. The frictional force is considerable and if the angle .alpha. is too small or if .mu. is sufficiently large, the wedge becomes locked in position and/or the movement of the device is irreversible. In the drum brake mechanisms referred to, the wedge is replaced by a pair of identical balls or rollers 3,4, as shown in FIG. 2, and the plunger 5 acts through them to apply a side thrust to the surfaces 6. Movement of the plunger by the force F forces the two surfaces 6 apart as before but in this arrangement the two balls or rollers roll upon each other and on the inclined surfaces 6 and there is sliding or skidding only between them and the plunger. This greatly reduces the friction forces, since the force F will be considerably smaller than the side thrust Q, and in addition the angle .delta. formed by the positive contact of the balls or rollers with each other and with the side faces 6 is half the wedge angle .alpha. so that a greater mechanical advantage is obtained. However, the arrangement disclosed is capable only of producing reciprocating movements and has no obvious application to the transmission of a continuous rotary motion.